Obtaining accurate data describing the shape and composition of objects located in a borehole has traditionally been a challenge, primarily due to the high temperatures and pressures within the borehole, the presence of associated opaque liquids, and significant space and maneuvering constraints. Furthermore, well operators need to obtain information as quickly as possible in order to minimize delays in operation, which can in some instances be extremely expensive. Known techniques for obtaining such images can be categorized in three basic classes: (1) those that use deformation of a mechanical probe; (2) those that use sound waves; and (3) those that use light.
In the first class, a mechanical probe or the like is deflected or deformed to create a representation of the borehole or a target object within the borehole. Typically, the probe is formed from a block of deformable material such as lead, which is lowered onto the target object and then retrieved. The resulting impression is interpreted to determine the shape of the object. However, since the impression must be brought to the surface for subsequent examination, this technique is time-consuming and does not easily admit to repeatable investigations.
For example, published U.S. Application No. 2009/0195647 A1 by Lynde discloses a technique in which a target object displaces a multitude of pins linked to sensors, and the measured displacements are then used to reconstruct the shape of a target object. This approach provides no information about the composition of the object, however.
Alternatively, U.S. Pat. No. 6,078,867 to Plumb et al. describes a system and associated method in which a multi-armed caliper scans a borehole, and the deflection of the arms is used to create a corresponding three-dimensional representation. Unfortunately, this method can only be used to look radially at the borehole walls or casing, and cannot be used to visualize objects disposed axially within the borehole.
In a second class, ultrasonic waves are emitted by a transducer, and the waves reflected from the object are recorded by receivers. For example, U.S. Pat. No. 4,847,814 to Anghern describes a system using multiple acoustic pulses to determine the distance to a borehole wall and thereby create a corresponding three-dimensional image. Similarly, U.S. Pat. No. 5,987,385 to Varsamis et al. discloses an acoustic logging tool that creates a circumferential image of a borehole during drilling. Published U.S. Application No. 2012/0127830 by Desai discloses a similar tool. While acoustic methods can provide quick feedback, they all have the drawbacks that any resulting data requires prior knowledge of the shape of the object; generally do not produce sufficiently clear images; and the results ultimately require expert interpretation.
In the third class, the target object is illuminated with electromagnetic radiation, typically visible light or x-rays, and radiation that is either reflected from or scattered by the object is used to create an image. For example, U.S. Pat. No. 6,678,050 to Pope et al. describes a technique for detecting and analyzing methane in coal bed methane formations using visible spectrum optical spectrometry, and published U.S. Application No. 2012/0169841 by Chemali et al. describes related optical tools and imaging methods. As a general matter, any technique using light at various optical wavelengths will suffer from distortion caused by the opacity of well fluids at those wavelengths. In order to obtain a clear image, the well fluids must therefore be replaced, which is a very costly and time-consuming operation.
Outside of the optical band, certain wavelengths of radiation can penetrate through the optically opaque fluids. For example, published U.S. Application No. 2010/0059219 by Roberts et al. describes a method that uses millimeter wavelength radiation to image target objects in a borehole. Unfortunately, millimeter wavelength imaging cannot provide information about the composition of the object.
Similarly, U.S. Pat. No. 3,564,251 to Youmans describes an apparatus in which x-rays scattered by the casing or the wall of an uncased borehole is recorded; U.S. Pat. No. 3,976,879 to Turcotte discloses a system that uses pulses of high energy photons to obtain information about the lithology of the earth formations surrounding a borehole; U.S. Pat. No. 8,138,471 to Shedlock et al. describes a device for inspecting wellbore casings and pipelines using a rotating pencil beam; and U.S. Pat. No. 4,883,956 to Melcher et al. discloses an apparatus and method for performing gamma-ray spectroscopy in a downhole environment. In all of these systems, however, the radiation is directed radially, and none are capable of visualizing an object located axially along the well.
In contrast, U.S. Pat. Nos. 7,675,029 and 7,705,294 to Ramstad et al. describe an apparatus and method for performing x-ray backscatter imaging in a fluid-carrying pipe. The x-ray imaging technique disclosed therein can produce images of objects located axially in the well, but affords very limited view depth and insufficient image clarity due to scattering by the well fluids.
While high energy radiation can penetrate through the optically opaque fluid to scatter from the target object, the radiation still scatters within the fluid and some of this scattering inevitably enters the detector. As the target object moves farther from the source and detector, an increasing amount of scattered radiation originating from the fluid enters the detector due to the increased volume of fluid both illuminated and viewed by the detector. Meanwhile, the amount of scattered radiation originating from the target object decreases due to the increased attenuation of the radiation incident upon the object. Consequently, objects become obscured as the distance between the x-ray source and object increases, thereby limiting the range of applicability of the methods.
There is, therefore, a longstanding but unmet need for an apparatus and method for obtaining a three-dimensional representation of a target object within a fluid-carrying conduit, such as a hydrocarbon exploration or production well, comprising means for overcoming the numerous technical deficiencies of the prior art.